Makefile: Correct dependency race condition with TPL

At present we sometimes see the following build error when building on a
machine with multiple cores.

+make[2]: *** No rule to make target 'dts/dt.dtb', needed by 'tpl/u-boot-tpl.dtb'.  Stop.

Add a dependency to correct this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>